For general cleaning of aluminum, it is best to use a mild detergent and water. For detergent, choose the same type of soap that you would use to clean your car. A more ambitious method is to buy or rent a power hose. If the shed is in your backyard, make sure that the detergent used is biodegradable. For dirt and stains that can not be cleaned with water and soap, try using some trisodium phosphate (TSP) and water. If there is any moss on the siding, add some bleach to the TSP. If none of these methods works, there are products that are designed to prepare aluminum and vinyl sidings for painting.